This invention relates to programmable logic integrated circuit devices, and more particularly to improved logic macrocells for such devices.
Pedersen U.S. Pat. No. 5,598,108 (hereby incorporated by reference herein) shows a macrocell for programmable logic devices which can form the sum of five product terms and output that sum, either directly or through a register. If a wider fan-in function is needed, the sum formed by one macrocell can be "borrowed" by a neighboring macrocell for combination (i.e., summation) with the five-product-term sum formed by the neighboring macrocell. When the product terms of a macrocell are thus borrowed by a neighboring macrocell, the other resources (e.g., the register) of the first macrocell become unusable. The potential waste of resources is even greater if less than all of the first macrocell's product term resources need to be borrowed.
It is known to make it possible for a macrocell to output one of its product terms (either registered or unregistered) if that product term is not needed for borrowing, even though other product terms of the macrocell are being borrowed. (See, for example, MACH 3 and 4 Family Data Book, 1993, Advanced Micro Devices, Inc., Sunnyvale, Calif.) However, no complex sum of products function can be implemented by a macrocell from which any of the product terms need to be borrowed.
In view of the foregoing, it is an object of this invention to increase the flexibility with which product terms of a macrocell can be borrowed by another macrocell or (if not borrowed) put to use in the macrocell of which they are nominally a part.
It is also known to feed back to the AND array of a programmable logic device a signal produced by a macrocell. This enables a macrocell to be used to implement a function that is needed by other macrocells to implement more complex functions. However, the ability to feed back only one signal produced by a macrocell may mean that other resources of the macrocell are wasted.
It is therefore another object of this invention to reduce or eliminate possible waste of macrocell resources due to macrocell architectures in which only one signal produced by a macrocell can be fed back to the AND array of the associated programmable logic device.